Lexical and Dynamic Scope

Lexical and dynamic scopes are normally used in Garbage Collector to indentify that if a variable is never used or expired.

Lexical Scope

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“Lexical” refers to the way code is written, so in lexical scope, the scope of a variable is determined by where it is defined in the source code.

var dog = 'alive'
 
function foo() {
     var cat = 'alive'
     console.log(dog)  // 'alive'
}
console.log(cat)  // out of scope

Dynamic Scope

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In dynamic scope, the lifetime of a variable is determined by the call stack, which means it depends not on where the variable is written in the code, but on how the program is executed.

Block Scope

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for (var i = 0; i < 3; i++) {
    setTimeout(() => console.log(i), 0);
}

The result of the code above is:

3
3
3

This may seem wrong at first, but it happens because the variable i has function scope, so each iteration uses the same i variable. When the loop finishes, i has the value 3, and all the setTimeout callbacks reference that same variable. To solve this problem, we use let instead of var to create block scope, which ensures that a different i is created for each iteration of the loop.

for (let i = 0; i<3; i++) {
    setTimeout(() => console.log(i), 0)
}

The result:

0
1
2

So basically, if you use var, it will reference only one i variable, and if you use let, it will reference a different i for each iteration.