“Use Strict” needed in a TypeScript file?

I've seen posts regarding where to put the "use strict" line in a TypeScript code file. My question is, why have it at all?

Since TypeScript is already a strongly typed language, what does "use strict" add?


  • 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,

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

// octal literals not allowed

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.

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.