What is `"use strict";`?

'use strict' is a statement used to enable strict mode to entire scripts or individual functions. Strict mode is a way to opt into 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 an exception (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, the benefits outweigh the disadvantages and there is not really a need to rely on the features that strict mode prohibits. We should all be using strict mode by default.

References

• Front End Interview Handbook