Hoisting in Strict Mode JavaScript

Hoisting in Strict Mode JavaScript

Strict mode ("use strict") changes a few hoisting-related behaviors in JavaScript. ES modules are strict by default.

What Strict Mode Changes

1. Assigning to an undeclared variable throws: Without strict mode, x = 5 creates a global var x. In strict mode, it throws ReferenceError: x is not defined.
2. this in regular functions is undefined: Not the global object. This affects hoisting of this in callbacks.
3. Duplicate parameter names throw: function foo(a, a) {} throws in strict mode.
4. Duplicate property names in objects: allowed in ES5 strict, throws in some later contexts.

What Strict Mode Does Not Change

• Hoisting itself still happens. var, let, const, and function declarations are still hoisted during the memory phase.
• The TDZ still applies to let and const.
• Function declarations are still hoisted with their full body.

Example: Undeclared Variable

"use strict";
function foo() {
  bar = 10; // ReferenceError: bar is not defined
  return bar;
}
foo();

Without strict mode, this would create window.bar = 10 silently.

Example: this in Strict Functions

"use strict";
function showThis() {
  console.log(this); // undefined
}
showThis();

Without strict mode, this would be window. This matters when a hoisted function is called before any binding.

ES Modules Are Strict by Default

You do not need "use strict" in a module. import and export automatically enable strict mode. Top-level this is undefined in modules.

Why This Matters

Strict mode surfaces bugs that sloppy mode hides. It prevents accidental globals, makes this predictable, and catches duplicate declarations. Combined with let/const, it gives you a safer hoisting story.

The Takeaway

Strict mode does not disable hoisting, but it makes related bugs visible. Undeclared assignments throw, this is undefined in regular functions, and duplicate parameters throw. ES modules are strict by default, so modern code gets these protections automatically.