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JavaScript Essential Training
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Using Strict Mode


From:

JavaScript Essential Training

with Simon Allardice

Video: Using Strict Mode

This next subject isn't strictly part of HTML5, but it is a forward-looking piece of JavaScript, so it belongs in the same discussion. You may come across references to something called Strict mode in JavaScript, particularly if you use code checkers like JSLint. Now, Strict mode is part of ECMAScript 5, but even though we're focused on ECMAScript 3 in this course, we might well be interested in it for our own code. It's a way to volunteer, to have your JavaScript to be held to higher standards than regular JavaScript, which has we've seen, will let you be sloppy.
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  1. 3m 28s
    1. Welcome
      1m 1s
    2. What you should know
      1m 44s
    3. Using the exercise files
      43s
  2. 15m 41s
    1. Introduction to JavaScript
      8m 6s
    2. Creating your first JavaScript
      2m 13s
    3. Getting to know the tools and applications
      5m 22s
  3. 56m 8s
    1. Understanding the structure of JavaScript code
      7m 9s
    2. Where to write your JavaScript
      3m 56s
    3. Creating variables
      6m 21s
    4. Working with conditional code
      5m 44s
    5. Working with operators
      13m 28s
    6. Sending messages to the console
      2m 59s
    7. Working with loops
      8m 1s
    8. Creating functions
      8m 30s
  4. 36m 13s
    1. Working with arrays
      7m 57s
    2. Working with numbers
      6m 13s
    3. Working with strings
      8m 27s
    4. Working with dates
      5m 38s
    5. Working with objects
      7m 58s
  5. 9m 6s
    1. What is the DOM?
      5m 49s
    2. Working with nodes and elements
      3m 17s
  6. 25m 17s
    1. Accessing DOM elements
      11m 3s
    2. Changing DOM elements
      5m 42s
    3. Creating DOM elements
      8m 32s
  7. 24m 45s
    1. Introduction to JavaScript event handling
      8m 16s
    2. Working with onClick and onLoad events
      7m 36s
    3. Working with onBlur and onFocus events
      2m 36s
    4. Working with timers
      6m 17s
  8. 21m 41s
    1. Common JavaScript errors
      7m 14s
    2. Using Firebug
      4m 7s
    3. Going through a debugging session
      10m 20s
  9. 10m 13s
    1. Accessing form elements
      4m 20s
    2. Preventing a form from being submitted
      2m 36s
    3. Hiding and showing form sections
      3m 17s
  10. 9m 49s
    1. CSS and JavaScript
      3m 46s
    2. Removing and applying CSS classes
      2m 16s
    3. Changing inline styles
      3m 47s
  11. 19m 44s
    1. Understanding JavaScript style
      7m 39s
    2. Minifying your code
      4m 28s
    3. Using JavaScript code checkers
      7m 37s
  12. 22m 24s
    1. Introduction to JavaScript libraries
      3m 17s
    2. Linking to multiple JavaScript files
      2m 11s
    3. Introduction to jQuery
      12m 7s
    4. Using a content distribution network to deliver JavaScript files
      4m 49s
  13. 17m 35s
    1. JavaScript in HTML5
      9m 37s
    2. Using Modernizr
      3m 2s
    3. Using Strict Mode
      4m 56s
  14. 33m 3s
    1. Knowing the JavaScript to avoid
      6m 35s
    2. Introduction to regular expressions
      6m 56s
    3. Working with AJAX
      10m 44s
    4. Working with objects and prototypes
      8m 48s
  15. 21m 10s
    1. Example: Countdown
      8m 3s
    2. Example: Resize
      5m 47s
    3. Example: Accordion
      7m 20s
  16. 4m 58s
    1. Where to go from here
      4m 0s
    2. Goodbye
      58s

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JavaScript Essential Training
5h 31m Beginner Jul 22, 2011

Viewers: in countries Watching now:

Use JavaScript to add new features and a richer, more compelling user interface on web pages. This course keeps current best practices and practical uses for JavaScript in mind, while covering syntax, working with the DOM, and developing and debugging across multiple platforms, devices, and browsers. Author Simon Allardice also shows how to progressively enhance and gracefully degrade web pages, and take advantage of the world of JavaScript libraries now available.

Topics include:
  • Understanding the structure of JavaScript code
  • Creating variables, functions, and loops
  • Writing conditional code
  • Sending messages to the console
  • Working with different variable types and objects
  • Creating and changing DOM objects
  • Event handling
  • Working with timers
  • Debugging JavaScript
  • Building smarter forms
  • Working with CSS, HTML5, and JavaScript
  • Using regular expressions
Subjects:
Developer Web Web Design Programming Languages Web Development
Software:
JavaScript
Author:
Simon Allardice

Using Strict Mode

This next subject isn't strictly part of HTML5, but it is a forward-looking piece of JavaScript, so it belongs in the same discussion. You may come across references to something called Strict mode in JavaScript, particularly if you use code checkers like JSLint. Now, Strict mode is part of ECMAScript 5, but even though we're focused on ECMAScript 3 in this course, we might well be interested in it for our own code. It's a way to volunteer, to have your JavaScript to be held to higher standards than regular JavaScript, which has we've seen, will let you be sloppy.

If you turn on Strict mode, for example, you must declare variables before you use them. If you fail to do so, that's an actual error. Your script will break. Strict mode will also cause an error if you define a function with multiple parameters with the same name. There are several more things it will complain about, a lot of them more advanced. And to volunteer for Strict mode you simply do this. At the top of any file of JavaScript you'd put the words "use strict" inside double quotes, semicolon as its own line.

Adding it up here means I'm saying turn on Strict mode for this entire file. You'd also add it as the first line of any function if you want to just do it function by function. But here's the thing, only certain browsers support Strict mode. Well, that's okay because browsers that don't understand it will simply look at this line, figure out it's a string literal by itself, and just ignore it. Browsers that do understand Strict mode, for example Firefox 4, will then be extra picky about your code.

So to prove that this works, I'm actually first going to just remove this, so we can run this code without Strict mode, and it's a very simple piece of code here that's just going to output the term "Hello world" here to this area of my HTML. And as I can see, Firebug isn't giving me any errors here. But if I jump back into the code and I turn on use strict mode, save that again, go back over into Firefox, and refresh this page, instantly we have a problem.

It starts off here saying, "property name samename appears more than once in an object literal." And bear in mind this is not just a diagnostic message; the script has broken. A moment ago it was actually changing the contents of this div and now it is not. So this line here where I am creating a new object and I've given it two properties with the same name, that was forgivable in regular JavaScript. It is not forgivable in Strict mode. So I'll fix that one, go back refresh it again.

Now it's giving me another problem, "duplicate formal argument A." It looks like in this function I've declared three parameters, but two of them have the same name, and that's not going to do at all. So I come back over here. Again, regular JavaScript would let me get away with that. It might have caused a logic problem, but it would still have attempted to work. Refresh it again, "assignment to undeclared variable foo," foo = "Hello world." That's because I'm not using var at the top here. So save that one, go back, test it again, and then finally, we're working correctly.

Now I'm a fan of Strict mode, so I actually tend to turn it on, but there are two things to be careful of. One, because it's common to combine multiple JavaScript files into one when you're trying to optimize performance, make sure that if you're putting "use strict" at the top of some of your files, you're not then mixing them up with some other files that don't use Strict. And bear in mind you can use Strict mode on a per-function basis. And if you know you're going to be combining multiple files together, some which use Strict mode and some which don't, you can combine the files with a blank function wrapper, which might look a little odd, but you do something like this, where I'm opening a parentheses in a function body at the top and I'm going to close it using the following format at the bottom, and then "use strict" effectively is the first line of this particular function.

This is actually what JSLint currently recommends as a way of doing it, but I think it's kind of clunky, but it's an option. But secondly, and more importantly, if you're going to be working with Strict mode, don't turn it on and then only check your code in browsers that don't support it, because bear in mind even using Strict mode, or turning it on, your code may still work in non-strict browsers and only break in Strict mode in a supported browser, so make sure you check it across multiple different browsers with the different capabilities.

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