Strict Mode

What it is

Strict mode is a slightly different, stricter dialect of JavaScript. It turns several silent mistakes into errors and removes a few features that were mistakes.

'use strict';

try {
  undeclared = 5;
} catch (error) {
  console.log(error.name + ': ' + error.message);
}

ReferenceError: undeclared is not defined

Without strict mode that line creates a global variable and carries on, which is how a typo in one function ends up changing something on the other side of a program.

What changes

• Assigning to an undeclared name throws instead of creating a global.
• Assigning to a read-only or frozen property throws instead of failing quietly.
• this is undefined in a plain function call rather than the global object.
• Duplicate parameter names are a syntax error.
• Octal literals written as 010 are a syntax error.
• delete on a plain variable is a syntax error.

'use strict';

const frozen = Object.freeze({ a: 1 });

try {
  frozen.a = 2;
} catch (error) {
  console.log(error.name);
}

TypeError

Outside strict mode that assignment does nothing at all and reports no problem, which is considerably worse than an error.

Where you already have it

You rarely need to write the directive today, because the places modern code lives are strict already.

Writing the directive

It must be the very first statement in the file or function, written as a plain string. Anything above it, including a stray semicolon, silently turns it off.

// Works: first line of the file.
'use strict';

// Does nothing: something came first.
const ready = true;
'use strict';