Front-end Job Interview Questions
==
Unlike typical software engineer job interviews, front-end job interviews have less emphasis on algorithms and have more questions on intricate knowledge and expertise about the domain — HTML, CSS, JavaScript, just to name a few areas.
While there are some existing resources to help front end developers in preparing for interviews, they aren't as abundant as materials for a software engineer interview. Among the existing resources, probably the most helpful question bank would be [Front-end Job Interview Questions](https://github.com/h5bp/Front-end-Developer-Interview-Questions). Unfortunately, I couldn't find many complete and satisfactory answers for these questions online, hence here is my attempt at answering them. Being an open source repository, the project can live on with the support of the community as the state of web evolves.
## Table of Contents
1. [HTML Questions](#html-questions)
1. [CSS Questions](#css-questions)
1. [JS Questions](#js-questions)
## HTML Questions
Answers to [Front-end Job Interview Questions - HTML Questions](https://github.com/h5bp/Front-end-Developer-Interview-Questions#html-questions). Pull requests for suggestions and corrections are welcome!
### What does a `doctype` do?
`doctype` is an abbreviation for document type. It is a declaration used in HTML5 to distinguish between a standards-compliant parsing mode and a quirks parsing mode. Hence its presence tells the browser to parse and render the webpage in standards mode.
Moral of the story - just add `` at the start of your page.
###### References
- https://stackoverflow.com/questions/7695044/what-does-doctype-html-do
- https://www.w3.org/QA/Tips/Doctype
### What's the difference between full standards mode, almost standards mode and quirks mode?
- **Quirks mode** - Layout emulates non-standard behavior in Netscape Navigator 4 and Internet Explorer 5. This is essential in order to support websites that were built before the widespread adoption of web standards. The list of quirks can be found [here](https://developer.mozilla.org/en-US/docs/Mozilla/Mozilla_quirks_mode_behavior).
- **Full standards mode** - The layout behavior is the one described by the HTML and CSS specifications.
- **Almost standards mode** - There are only a very small number of quirks implemented. Differences can be found [here](https://developer.mozilla.org/en-US/docs/Gecko's_Almost_Standards_Mode).
###### References
- https://developer.mozilla.org/en-US/docs/Quirks_Mode_and_Standards_Mode
### What's the difference between HTML and XHTML?
XHTML belongs to the family of XML markups languages and is different from HTML. Some of differences are as follows:
- XHTML documents have to be well-formed, unlike HTML, which is more forgiving.
- XHTML is case-sensitive for element and attribute names, while HTML is not.
- Raw `<` and `&` characters are not allowed except inside of `CDATA` Sections (``). JavaScript typically contains characters which can not exist in XHTML outside of CDATA Sections, such as the `<` operator. Hence it is tricky to use inline `styles` or `script` tags in XHTML and should be avoided.
- A fatal parse error in XML (such as an incorrect tag structure) causes document processing to be aborted.
Full list of differences can be found on [Wikipedia](https://en.wikipedia.org/wiki/XHTML#Relationship_to_HTML).
###### References
- https://developer.mozilla.org/en-US/docs/Archive/Web/Properly_Using_CSS_and_JavaScript_in_XHTML_Documents_
- https://en.wikipedia.org/wiki/XHTML
### Are there any problems with serving pages as `application/xhtml+xml`?
Basically the problems lie in the differences between parsing HTML and XML as mentioned above.
- XHTML, or rather, XML syntax is less forgiving and if your page isn't fully XML-compliant, there will be parsing errors and users get unreadable content.
- Serving your pages as `application/xhtml+xml` will cause Internet Explorer 8 to show a download dialog box for an unknown format instead of displaying your page, as the first version of Internet Explorer with support for XHTML is Internet Explorer 9.
###### References
- https://developer.mozilla.org/en-US/docs/Quirks_Mode_and_Standards_Mode#XHTML
### How do you serve a page with content in multiple languages?
The question is a little vague, I will assume that it is asking about the most common case, which is how to serve a page with content available in multiple languages, but the content within the page should be displayed only in one consistent language.
When an HTTP request is made to a server, the requesting user agent usually sends information about language preferences, such as in the `Accept-Language` header. The server can then use this information to return a version of the document in the appropriate language if such an alternative is available. The returned HTML document should also declare the `lang` attribute in the `` tag, such as `...`.
In the back end, the HTML markup will contain `i18n` placeholders and content for the specific language stored in YML or JSON formats. The server then dynamically generates the HTML page with content in that particular language, usually with the help of a back end framework.
###### References
- https://www.w3.org/International/getting-started/language
### What kind of things must you be wary of when designing or developing for multilingual sites?
- Use `lang` attribute in your HTML.
- Directing users to their native language - Allow a user to change his country/language easily without hassle.
- Text in images is not a scalable approach - Placing text in an image is still a popular way to get good-looking, non-system fonts to display on any computer. However to translate image text, each string of text will need to have it's a separate image created for each language. Anything more than a handful of replacements like this can quickly get out of control.
- Restrictive words / sentence length - Some content can be longer when written in another language. Be wary of layout or overflow issues in the design. It's best to avoid designing where the amount of text would make or break a design. Character counts come into play with things like headlines, labels, and buttons. They are less of an issue with free flowing text such as body text or comments.
- Be mindful of how colors are perceived - Colors are perceived differently across languages and cultures. The design should use color appropriately.
- Formatting dates and currencies - Calendar dates are sometimes presented in different ways. Eg. "May 31, 2012" in the U.S. vs. "31 May 2012" in parts of Europe.
- Do not concatenate translated strings - Do not do anything like `"The date today is " + date`. It will break in languages with different word order. Using template parameters instead.
- Language reading direction - In English, we read from left-to-right, top-to-bottom, in traditional Japanese, text is read up-to-down, right-to-left.
###### References
- https://www.quora.com/What-kind-of-things-one-should-be-wary-of-when-designing-or-developing-for-multilingual-sites
### What are `data-` attributes good for?
Before JavaScript frameworks became popular, front end developers used `data-` attributes to store extra data within the DOM itself, without other hacks such as non-standard attributes, extra properties on the DOM. It is intended to store custom data private to the page or application, for which there are no more appropriate attributes or elements.
These days, using `data-` attributes is not encouraged. One reason is that users can modify the data attribute easily by using inspect element in the browser. The data model is better stored within JavaScript itself and stay updated with the DOM via data binding possibly through a library or a framework.
###### References
- http://html5doctor.com/html5-custom-data-attributes/
- https://www.w3.org/TR/html5/dom.html#embedding-custom-non-visible-data-with-the-data-*-attributes
### Consider HTML5 as an open web platform. What are the building blocks of HTML5?
- Semantics - Allowing you to describe more precisely what your content is.
- Connectivity - Allowing you to communicate with the server in new and innovative ways.
- Offline and storage - Allowing webpages to store data on the client-side locally and operate offline more efficiently.
- Multimedia - Making video and audio first-class citizens in the Open Web.
- 2D/3D graphics and effects - Allowing a much more diverse range of presentation options.
- Performance and integration - Providing greater speed optimization and better usage of computer hardware.
- Device access - Allowing for the usage of various input and output devices.
- Styling - Letting authors write more sophisticated themes.
###### References
- https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/HTML5
### Describe the difference between a `cookie`, `sessionStorage` and `localStorage`.
All the above mentioned technologies are key-value storage mechanisms on the client side. They are only able to store values as strings.
| |`cookie`|`localStorage`|`sessionStorage`|
|--|--|--|--|
| Initiator | Client or server. Server can use `Set-Cookie` header | Client | Client |
| Expiry | Manually set | Forever | On tab close |
| Persistent across browser sessions | Depends on whether expiration is set | Yes | No |
| Have domain associated | Yes | No | No |
| Sent to server with every HTTP request| Cookies are automatically being sent via `Cookie` header | No | No |
| Capacity (per domain) | 4kb | 5MB | 5MB |
| Accessibility | Any window | Any window | Same tab |
###### References
- https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies
- http://tutorial.techaltum.com/local-and-session-storage.html
### Describe the difference between `
```
```js
// File loaded from https://example.com?callback=printData
printData({ name: 'Yang Shun' });
```
The client has to have the `printData` function in its global scope and the function will be executed by the client when the response from the cross-origin domain is received.
JSONP can be unsafe and has some security implications. As JSONP is really JavaScript, it can do everything else JavaScript can do, so you need to trust the provider of the JSONP data.
These days, [CORS](http://en.wikipedia.org/wiki/Cross-origin_resource_sharing) is the recommended approach and JSONP is seen as a hack.
###### References
- https://stackoverflow.com/a/2067584/1751946
### Have you ever used JavaScript templating? If so, what libraries have you used?
Yes. Handlebars, Underscore, Lodash, AngularJS and JSX. I disliked templating in AngularJS because it made heavy use of strings in the directives and typos would go uncaught. JSX is my new favourite as it is closer to JavaScript and there is barely any syntax to learn. Nowadays, you can even use ES2015 template string literals as a quick way for creating templates without relying on third-party code.
```js
const template = `
My name is: ${name}
`;
```
However, do be aware of a potential XSS in the above approach as the contents are not escaped for you, unlike in templating libraries.
### Explain "hoisting".
Hoisting is a term used to explain the behavior of variable declarations in your code. Variables declared or initialized with the `var` keyword will have their declaration "hoisted" up to the top of the current scope. However, only the declaration is hoisted, the assignment (if there is one), will stay where it is. Let's explain with a few examples.
```js
// var declarations are hoisted.
console.log(foo); // undefined
var foo = 1;
console.log(foo); // 1
// let/const declarations are NOT hoisted.
console.log(bar); // ReferenceError: bar is not defined
let bar = 2;
console.log(bar); // 2
```
Function declarations have the body hoisted while the function expressions (written in the form of variable declarations) only has the variable declaration hoisted.
```js
// Function Declaration
console.log(foo); // [Function: foo]
foo(); // 'FOOOOO'
function foo() {
console.log('FOOOOO');
}
console.log(foo); // [Function: foo]
// Function Expression
console.log(bar); // undefined
bar(); // Uncaught TypeError: bar is not a function
var bar = function() {
console.log('BARRRR');
}
console.log(bar); // [Function: bar]
```
### Describe event bubbling.
When an event triggers on a DOM element, it will attempt to handle the event if there is a listener attached, then the event is bubbled up to its parent and the same thing happens. This bubbling occurs up the element's ancestors all the way to the `document`. Event bubbling is the mechanism behind event delegation.
### What's the difference between an "attribute" and a "property"?
Attributes are defined on the HTML markup but properties are defined on the DOM. To illustrate the difference, imagine we have this text field in our HTML: ``.
```js
const input = document.querySelector('input');
console.log(input.getAttribute('value')); // Hello
console.log(input.value); // Hello
```
But after you change the value of the text field by adding "World!" to it, this becomes:
```js
console.log(input.getAttribute('value')); // Hello
console.log(input.value); // Hello World!
```
###### References
- https://stackoverflow.com/questions/6003819/properties-and-attributes-in-html
### Why is extending built-in JavaScript objects not a good idea?
Extending a built-in/native JavaScript object means adding properties/functions to its `prototype`. While this may seem like a good idea at first, it is dangerous in practice. Imagine your code uses a few libraries that both extend the `Array.prototype` by adding the same `contains` method, the implementations will overwrite each other and your code will break if the behavior of these two methods are not the same.
The only time you may want to extend a native object is when you want to create a polyfill, essentially providing your own implementation for a method that is part of the JavaScript specification but might not exist in the user's browser due to it being an older browser.
###### References
- http://lucybain.com/blog/2014/js-extending-built-in-objects/
### Difference between document load event and document DOMContentLoaded event?
The `DOMContentLoaded` event is fired when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading.
`window`'s `load` event is only fired after the DOM and all dependent resources and assets have loaded.
###### References
- https://developer.mozilla.org/en-US/docs/Web/Events/DOMContentLoaded
- https://developer.mozilla.org/en-US/docs/Web/Events/load
### What is the difference between `==` and `===`?
`==` is the abstract equality operator while `===` is the strict equality operator. The `==` operator will compare for equality after doing any necessary type conversions. The `===` operator will not do type conversion, so if two values are not the same type `===` will simply return `false`. When using `==`, funky things can happen, such as:
```js
1 == '1' // true
1 == [1] // true
1 == true // true
0 == '' // true
0 == '0' // true
0 == false // true
```
My advice is never to use the `==` operator, except for convenience when comparing against `null` or `undefined`, where `a == null` will return `true` if `a` is `null` or `undefined`.
```js
var a = null;
console.log(a == null); // true
console.log(a == undefined); // true
```
###### References
- https://stackoverflow.com/questions/359494/which-equals-operator-vs-should-be-used-in-javascript-comparisons
### Explain the same-origin policy with regards to JavaScript.
The same-origin policy prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. This policy prevents a malicious script on one page from obtaining access to sensitive data on another web page through that page's Document Object Model.
###### References
- https://en.wikipedia.org/wiki/Same-origin_policy
### Make this work:
```js
duplicate([1,2,3,4,5]); // [1,2,3,4,5,1,2,3,4,5]
```
```js
function duplicate(arr) {
return arr.concat(arr);
}
duplicate([1,2,3,4,5]); // [1,2,3,4,5,1,2,3,4,5]
```
### Why is it called a Ternary expression, what does the word "Ternary" indicate?
"Ternary" indicates three, and a ternary expression accepts three operands, the test condition, the "then" expression and the "else" expression. Ternary expressions are not specific to JavaScript and I'm not sure why it is even in this list.
###### References
- https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Conditional_Operator
### What is `"use strict";`? What are the advantages and disadvantages to using it?
'use strict' is a statement used to enable strict mode to entire scripts or individual functions. Strict mode is a way to opt in to a restricted variant of JavaScript.
Advantages:
- Makes it impossible to accidentally create global variables.
- Makes assignments which would otherwise silently fail to throw an exception.
- Makes attempts to delete undeletable properties throw (where before the attempt would simply have no effect).
- Requires that function parameter names be unique.
- `this` is undefined in the global context.
- It catches some common coding bloopers, throwing exceptions.
- It disables features that are confusing or poorly thought out.
Disadvantages:
- Many missing features that some developers might be used to.
- No more access to `function.caller` and `function.arguments`.
- Concatenation of scripts written in different strict modes might cause issues.
Overall, I think the benefits outweigh the disadvantages, and I never had to rely on the features that strict mode blocks. I would recommend using strict mode.
###### References
- http://2ality.com/2011/10/strict-mode-hatred.html
- http://lucybain.com/blog/2014/js-use-strict/
### Create a for loop that iterates up to `100` while outputting **"fizz"** at multiples of `3`, **"buzz"** at multiples of `5` and **"fizzbuzz"** at multiples of `3` and `5`.
Check out this version of FizzBuzz by [Paul Irish](https://gist.github.com/jaysonrowe/1592432#gistcomment-790724).
```js
for (let i = 1; i <= 100; i++) {
let f = i % 3 == 0, b = i % 5 == 0;
console.log(f ? (b ? 'FizzBuzz' : 'Fizz') : (b ? 'Buzz' : i));
}
```
I would not advise you to write the above during interviews though. Just stick with the long but clear approach. For more wacky versions of FizzBuzz, check out the reference link below.
###### References
- https://gist.github.com/jaysonrowe/1592432
### Why is it, in general, a good idea to leave the global scope of a website as-is and never touch it?
Every script has access to the global scope, and if everyone is using the global namespace to define their own variables, there will bound to be collisions. Use the module pattern (IIFEs) to encapsulate your variables within a local namespace.
### Why would you use something like the `load` event? Does this event have disadvantages? Do you know any alternatives, and why would you use those?
The `load` event fires at the end of the document loading process. At this point, all of the objects in the document are in the DOM, and all the images, scripts, links and sub-frames have finished loading.
The DOM event `DOMContentLoaded` will fire after the DOM for the page has been constructed, but do not wait for other resources to finish loading. This is preferred in certain cases when you do not need the full page to be loaded before initializing.
TODO.
###### References
- https://developer.mozilla.org/en-US/docs/Web/API/GlobalEventHandlers/onload
### Explain what a single page app is and how to make one SEO-friendly.
The below is taken from the awesome [Grab Front End Guide](https://github.com/grab/front-end-guide), which coincidentally, is written by me!
Web developers these days refer to the products they build as web apps, rather than websites. While there is no strict difference between the two terms, web apps tend to be highly interactive and dynamic, allowing the user to perform actions and receive a response for their action. Traditionally, the browser receives HTML from the server and renders it. When the user navigates to another URL, a full-page refresh is required and the server sends fresh new HTML for the new page. This is called server-side rendering.
However in modern SPAs, client-side rendering is used instead. The browser loads the initial page from the server, along with the scripts (frameworks, libraries, app code) and stylesheets required for the whole app. When the user navigates to other pages, a page refresh is not triggered. The URL of the page is updated via the [HTML5 History API](https://developer.mozilla.org/en-US/docs/Web/API/History_API). New data required for the new page, usually in JSON format, is retrieved by the browser via [AJAX](https://developer.mozilla.org/en-US/docs/AJAX/Getting_Started) requests to the server. The SPA then dynamically updates the page with the data via JavaScript, which it has already downloaded in the initial page load. This model is similar to how native mobile apps work.
The benefits:
- The app feels more responsive and users do not see the flash between page navigations due to full-page refreshes.
- Fewer HTTP requests are made to the server, as the same assets do not have to be downloaded again for each page load.
- Clear separation of the concerns between the client and the server; you can easily build new clients for different platforms (e.g. mobile, chatbots, smart watches) without having to modify the server code. You can also modify the technology stack on the client and server independently, as long as the API contract is not broken.
The downsides:
- Heavier initial page load due to loading of framework, app code, and assets required for multiple pages.
- There's an additional step to be done on your server which is to configure it to route all requests to a single entry point and allow client-side routing to take over from there.
- SPAs are reliant on JavaScript to render content, but not all search engines execute JavaScript during crawling, and they may see empty content on your page. This inadvertently hurts the Search Engine Optimization (SEO) of your app. However, most of the time, when you are building apps, SEO is not the most important factor, as not all the content needs to be indexable by search engines. To overcome this, you can either server-side render your app or use services such as [Prerender](https://prerender.io/) to "render your javascript in a browser, save the static HTML, and return that to the crawlers".
###### References
- https://github.com/grab/front-end-guide#single-page-apps-spas
- http://stackoverflow.com/questions/21862054/single-page-app-advantages-and-disadvantages
- http://blog.isquaredsoftware.com/presentations/2016-10-revolution-of-web-dev/
- https://medium.freecodecamp.com/heres-why-client-side-rendering-won-46a349fadb52
### What is the extent of your experience with Promises and/or their polyfills?
Possess working knowledge of it. A promise is an object that may produce a single value some time in the future: either a resolved value, or a reason that it's not resolved (e.g., a network error occurred). A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.
Some common polyfills are `$.deferred`, Q and Bluebird but not all of them comply to the specification. ES2015 supports Promises out of the box and polyfills are typically not needed these days.
###### References
- https://medium.com/javascript-scene/master-the-javascript-interview-what-is-a-promise-27fc71e77261
### What are the pros and cons of using Promises instead of callbacks?
**Pros**
- Avoid callback hell which can be unreadable.
- Makes it easy to write sequential asynchronous code that is readable with `.then()`.
- Makes it easy to write parallel asynchronous code with `Promise.all()`.
**Cons**
- Slightly more complex code (debatable).
- In older browsers where ES2015 is not supported, you need to load a polyfill in order to use it.
### What are some of the advantages/disadvantages of writing JavaScript code in a language that compiles to JavaScript?
Some examples of languages that compile to JavaScript include CoffeeScript, Elm, ClojureScript, PureScript and TypeScript.
Advantages:
- Fixes some of the longstanding problems in JavaScript and discourages JavaScript anti-patterns.
- Enables you to write shorter code, by providing some syntactic sugar on top of JavaScript, which I think ES5 lacks, but ES2015 is awesome.
- Static types are awesome (in the case of TypeScript) for large projects that need to be maintained over time.
Disadvantages:
- Require a build/compile process as browsers only run JavaScript and your code will need to be compiled into JavaScript before being served to browsers.
- Debugging can be a pain if your source maps do not map nicely to your pre-compiled source.
- Most developers are not familiar with these languages and will need to learn it. There's a ramp up cost involved for your team if you use it for your projects.
- Smaller community (depends on the language), which means resources, tutorials, libraries and tooling would be harder to find.
- IDE/editor support might be lacking.
- These languages will always be behind the latest JavaScript standard.
- Developers should be cognizant of what their code is being compiled to — because that is what would actually be running, and that is what matters in the end.
Practically, ES2015 has vastly improved JavaScript and made it much nicer to write. I don't really see the need for CoffeeScript these days.
###### References
- https://softwareengineering.stackexchange.com/questions/72569/what-are-the-pros-and-cons-of-coffeescript
### What tools and techniques do you use for debugging JavaScript code?
- React and Redux
- [React Devtools](https://github.com/facebook/react-devtools)
- [Redux Devtools](https://github.com/gaearon/redux-devtools)
- JavaScript
- [Chrome Devtools](https://hackernoon.com/twelve-fancy-chrome-devtools-tips-dc1e39d10d9d)
- `debugger` statement
- Good old `console.log` debugging
###### References
- https://hackernoon.com/twelve-fancy-chrome-devtools-tips-dc1e39d10d9d
- https://raygun.com/blog/javascript-debugging/
### What language constructions do you use for iterating over object properties and array items?
For objects:
- `for` loops - `for (var property in obj) { console.log(property); }`. However, this will also iterate through its inherited properties, and you will add an `obj.hasOwnProperty(property)` check before using it.
- `Object.keys()` - `Object.keys(obj).forEach(function (property) { ... })`. `Object.keys()` is a static method that will lists all enumerable properties of the object that you pass it.
- `Object.getOwnPropertyNames()` - `Object.getOwnPropertyNames(obj).forEach(function (property) { ... })`. `Object.getOwnPropertyNames()` is a static method that will lists all enumerable and non-enumerable properties of the object that you pass it.
For arrays:
- `for` loops - `for (var i = 0; i < arr.length; i++)`. The common pitfall here is that `var` is in the function scope and not the block scope and most of the time you would want block scoped iterator variable. ES2015 introduces `let` which has block scope and it is recommended to use that instead. So this becomes: `for (let i = 0; i < arr.length; i++)`.
- `forEach` - `arr.forEach(function (el, index) { ... })`. This construct can be more convenient at times because you do not have to use the `index` if all you need is the array elements. There are also the `every` and `some` methods which will allow you to terminate the iteration early.
Most of the time, I would prefer the `.forEach` method, but it really depends on what you are trying to do. `for` loops allow more flexibility, such as prematurely terminate the loop using `break` or incrementing the iterator more than once per loop.
### Explain the difference between mutable and immutable objects.
- What is an example of an immutable object in JavaScript?
- What are the pros and cons of immutability?
- How can you achieve immutability in your own code?
TODO
### Explain the difference between synchronous and asynchronous functions.
Synchronous functions are blocking while asynchronous functions are not. In synchronous functions, statements complete before the next statement is run. In this case the program is evaluated exactly in order of the statements and execution of the program is paused if one of the statements take a very long time.
Asynchronous functions usually accept a callback as a parameter and execution continues on the next line immediately after the asynchronous function is invoked. The callback is only invoked when the asynchronous operation is complete and the call stack is empty. Heavy duty operations such as loading data from a web server or querying a database should be done asynchronously so that the main thread can continue executing other operations instead of blocking until that long operation to complete (in the case of browsers, the UI will freeze).
### What is event loop? What is the difference between call stack and task queue?
The event loop is a single-threaded loop that monitors the call stack and checks if there is any work to be done in the task queue. If the call stack is empty and there are callback functions in the task queue, a function is dequeued and pushed onto the call stack to be executed.
If you haven't already checked out Philip Robert's [talk on the Event Loop](https://2014.jsconf.eu/speakers/philip-roberts-what-the-heck-is-the-event-loop-anyway.html), you should. It is one of the most viewed videos on JavaScript.
###### References
- https://2014.jsconf.eu/speakers/philip-roberts-what-the-heck-is-the-event-loop-anyway.html
- http://theproactiveprogrammer.com/javascript/the-javascript-event-loop-a-stack-and-a-queue/
### Explain the differences on the usage of `foo` between `function foo() {}` and `var foo = function() {}`
The former is a function declaration while the latter is a function expression. The key difference is that function declarations have its body hoisted but the bodies of function expressions are not (they have the same hoisting behaviour as variables). For more explanation on hoisting, refer to the question above on hoisting. If you try to invoke a function expression before it is defined, you will get an `Uncaught TypeError: XXX is not a function` error.
**Function Declaration**
```js
foo(); // 'FOOOOO'
function foo() {
console.log('FOOOOO');
}
```
**Function Expression**
```js
foo(); // Uncaught TypeError: foo is not a function
var foo = function() {
console.log('FOOOOO');
}
```
###### References
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function
### Other Answers
- http://flowerszhong.github.io/2013/11/20/javascript-questions.html