"Use Strict" needed in a TypeScript file?

Updates

TypeScript 1.8+: "use strict"; is emitted in modules (Read more).

TypeScript 2.1+: --alwaysStrict compiler option parses all files in strict mode and emits "use strict" at the top of all outputted files (Read more).

You can find a list of some examples by searching TypeScript's tests for "in strict mode".

Here's some examples of code that will only throw a compile time error when you "use strict";:

// future reserved keyword not allowed as variable name
var let,
    yield,
    public,
    private,
    protected,
    static,
    implements;

// "delete" cannot be called on an identifier
var a;
delete a;

// octal literals not allowed
03;

There are a few more examples where "use strict"; would throw an error only at runtime. For example:

"use strict";
delete Object.prototype;

Personally, I don't find it all that useful at preventing me from making mistakes in TypeScript and the additional noise it adds to a file makes me not bother writing it. That said, starting in TS 2.1 I'll enable the --alwaysStrict compiler option because it adds the slight additional strictness without any code maintenance overhead.

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For my money, yes, "use strict"; should be included in TypeScript files.

Disregarding the compile time effects of "use strict"; on TypeScript, there is likely a runtime impact when the generated JavaScript is executed:

MDN identifies performance improvements in avoiding boxing this in function calls, and the removal of the function.caller and function.arguments properties.

Jeff Walden of Mozilla has also hinted at opportunities for performance gains in this answer.

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