Why was block scope not originally implemented in JavaScript?

Question

I have read, and discovered through my own experience, that JavaScript doesn\'t have a block scope. Assuming that the language was designed this way for a reason, can you ex

Answer 1

There are many reasons, but some that come to mind are to aid in parsing/debugging code that uses object literals (which can sometimes look like a block), and to simplify the garbage collection of local variables.

I hope that the promised support (discussed here, for example, http://esdiscuss.org/notes/2012-07-25) turns out to be real because it would be very convenient to use variables like i that were local to only a single loop.

Answer 2

^{Converting my comment to answer}

Choice of the creator: I tweeted Brendan and got the following answer:

@mplungjan 10 days did not leave time for block scope. Also many "scripting languages" of that mid-90s era had few scopes & grew more later.

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That said, here are some relevant points:

• Block statement

Important: JavaScript prior to ECMAScript2015 (6th edition) does not have block scope. Variables introduced within a block are scoped to the containing function or script, and the effects of setting them persist beyond the block itself. In other words, block statements do not introduce a scope. Although "standalone" blocks are valid syntax, you do not want to use standalone blocks in JavaScript, because they don't do what you think they do, if you think they do anything like such blocks in C or Java.

• a workaround

we can artificially introduce scopes by creating new functions and immediately invoking them

• ECMAScript 2015 (ES6)

let and const declared variables are hoisted but they are not initialized to undefined in the same way var is. Hence, referencing a let or const declared variable before its value is assigned raises a ReferenceError.

Redeclaration of the same variable in the same block scope raises a SyntaxError.

Answer 3

New answer as of 2015. ES6 does have block scope for variable definitions with the let and const keywords.

Answer 4

Block scope was not implemented for the following reasons:

1. It's makes the language easier to implement. JavaScript was initially designed as a language for writing interactive web applications. Hence it needed to be small and easy to implement.
2. Block scopes introduce a performance hit to dynamic languages like JavaScript. This is because when you try to access some variable which is not in the current scope JavaScript first checks the current scope, then the parent scope and so on until it either finds the variable or reaches the end. Hence the introduction of block scopes would make variable access in loops and nested loops very slow.
3. The lack of block scopes makes it easier to write programs. For example say you want to create a variable only if a certain condition is true. All you need to do in JavaScript is declare and define the variable within an if statement. In languages like C you would have to declare the variable outside the if statement and define it within the if statement.
4. The lack of block scopes allow declarations to be hoisted. This is especially useful in the case of function declarations. For example see this fiddle: http://jsfiddle.net/L6SgM/ (note however that this example doesn't work in Firefox).
5. Since JavaScript supports first-class function expressions we don't need block scopes. They can be simulated using immediately invoked function expressions.