Default binding is the rule that applies when a function is called as a standalone function — not as a method, not with new, not with call/apply/bind. In this case, this refers to the global object (or undefined in strict mode).

When you call a function without any context, JavaScript falls back to the default binding:

function showThis() {
  console.log(this);
}

showThis();
// Browser: Window { ... }
// Node.js: global { ... }
// Strict mode: undefined

function nonStrict() {
  console.log(this); // Window / global
}

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

Default binding is the lowest priority rule. It only applies when no other rule matches. Understanding when it applies helps you avoid bugs where this unexpectedly refers to the global object.

// 1. Direct function call
function logThis() {
  console.log(this);
}
logThis(); // default binding → global

// 2. Callback (non-strict)
function User(name) {
  this.name = name;
}
User.prototype.sayHi = function() {
  setTimeout(function() {
    console.log(`Hi, I'm ${this.name}`); // this = global → undefined
  }, 100);
};

// 3. Function expression
const greet = function() {
  console.log(this);
};
greet(); // default binding → global

• Use 'use strict' to avoid accidental global this
• Use arrow functions when you want to avoid default binding
• Never rely on default binding in your code

• Default binding applies to standalone function calls
• this = global object (non-strict) or undefined (strict)
• Lowest priority binding rule
• Avoid relying on default binding in production code

• MDN: this
• Back to Overview