var, let and const

When it comes to backwards compatibility for variable declarations (var, let, and const) in browsers, here's a detailed breakdown of their support across different versions:

1. `var`

Introduced in: ECMAScript 5 (ES5) — 2009
Compatibility: var has been supported since very early versions of JavaScript, and all browsers support it, including Internet Explorer (IE 6+).
Behavior:
- Global scope: Variables declared with var are scoped to the function (or globally if declared outside a function).
- Hoisting: Variables declared with var are hoisted to the top of their scope, meaning they can be accessed before their declaration (though their value will be undefined until the declaration line is reached).

2. `let`

Introduced in: ECMAScript 6 (ES6) — 2015
Compatibility:
- Supported in modern browsers like Chrome, Firefox, Edge, Safari, and Opera (since 2015, around version 49+ in Chrome).
- Not supported in: Internet Explorer (IE 11 and earlier).
Behavior:
- Block-scoped: Variables declared with let are scoped to the nearest enclosing block (e.g., inside loops or conditionals).
- No hoisting: While let is hoisted, it is not initialized until the line of declaration. If accessed before the declaration, it results in a ReferenceError (often referred to as the "temporal dead zone").

3. `const`

Introduced in: ECMAScript 6 (ES6) — 2015
Compatibility:
- Supported in modern browsers like Chrome, Firefox, Edge, Safari, and Opera (since 2015, around version 49+ in Chrome).
- Not supported in: Internet Explorer (IE 11 and earlier).
Behavior:
- Block-scoped: Like let, const is scoped to the nearest block.
- Immutability: Variables declared with const cannot be reassigned after initialization. However, the contents of objects and arrays (declared with const) are mutable.
- No hoisting: Similar to let, const is hoisted but in a "temporal dead zone," so trying to access it before declaration results in a ReferenceError.

Summary of Compatibility:

Feature	IE 6-8	IE 9-10	IE 11	Chrome (from)	Firefox (from)	Safari (from)	Edge (from)	Opera (from)
`var`	Yes	Yes	Yes	Yes (All)	Yes (All)	Yes (All)	Yes (All)	Yes (All)
`let`	No	No	No	Yes (49)	Yes (54)	Yes (10)	Yes (12)	Yes (36)
`const`	No	No	No	Yes (49)	Yes (54)	Yes (10)	Yes (12)	Yes (36)

Key:

IE 6-8: Internet Explorer versions 6 to 8.
IE 9-10: Internet Explorer versions 9 and 10.
IE 11: Internet Explorer version 11.
Chrome (from): The first Chrome version that supports the feature.
Firefox (from): The first Firefox version that supports the feature.
Safari (from): The first Safari version that supports the feature.
Edge (from): The first Edge version that supports the feature.
Opera (from): The first Opera version that supports the feature.

Conclusion:

var is supported in all browsers, even older versions like Internet Explorer 6+.
let and const are not supported in Internet Explorer (any version) and are supported in modern browsers (Chrome, Firefox, Safari, Edge, Opera) from 2015 onwards.
If you need to support older browsers like IE, it’s safer to stick with var. For modern development, using let and const is recommended due to their block-scoping and immutability benefits.

Last modified: 02 February 2025