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42 lines
1 KiB
Markdown
42 lines
1 KiB
Markdown
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---
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id: c5fef6b1-c659-42ef-8b51-7657df6743e8
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title: JavaScript Strict Mode
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---
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# Description
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[Strict
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mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode)
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makes several changes to normal JavaScript semantics:
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- Eliminates some JavaScript silent errors by changing them to throw
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errors.
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- Fixes mistakes that make it difficult for JavaScript engines to
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perform optimizations: strict mode code can sometimes be made to run
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faster than identical code that's not strict mode.
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- Prohibits some syntax likely to be defined in future versions of
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ECMAScript.
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``` javascript
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function canYouSpotTheProblem() {
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"use strict";
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for (counter = 0; counter < 10; counter++) {
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console.log("Happy happy");
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}
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}
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canYouSpotTheProblem();
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// → ReferenceError: counter is not defined
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```
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In strict mode the \`this\` binding holds the value \`undefined\` in
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functions that are not called as methods:
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``` javascript
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"use strict";
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function doSomething() {
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console.log(this)
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}
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```
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