Are 'const' and 'let' interchangeable in Typescript?

Question

Are 'const' and 'let' interchangeable in Typescript?

Exploring AngularJS 2 and Typescript led me to create something using these technologies as a way to grasp the basics of Typescript. Through various sources, I delved into modules, Typescript concepts, with one particularly interesting topic discussing the differences between 'let' and 'var' when declaring variables. According to a query on Stack Overflow, the Typescript code snippet below was expected to show only one alert and throw an error in the console:

test.ts:

for(let i = 0; i < 1; i++) {
    alert(i);
}
alert(i);

Compiled test.js:

for(var i = 0; i < 1; i++) {
    alert(i);
}
alert(i);
//# sourceMappingURL=test.js.map

However, contrary to expectations, the compiler seems to ignore the "let" keyword and instead converts it to "var". This raises the question of why this discrepancy exists. Does Typescript function correctly only within classes?

My setup involves AngularJS configuration for 'npm start', which automatically compiles my 'test.ts' file:

  "scripts": {
    "start": "tsc && concurrently \"npm run tsc:w\" \"npm run lite\" ",
    "lite": "lite-server",
    "postinstall": "typings install",
    "tsc": "tsc",
    "tsc:w": "tsc -w",
    "typings": "typings"
  },

javascript typescript var let

Answer 1

Answer №1

However, it's not that simple. The compiler essentially translates the "let" command to a "var" command. Why does this mechanism exist? Is Typescript optimized for use with classes only?

The compiler defaults to transpiling to ES3, where the let keyword is not supported. Therefore, the emitter has to generate code using ES3-compatible syntax, hence replacing let with var.

If you prefer the output to use the let keyword, you need to switch to targeting ES6—by setting "target": "es6" in your tsconfig.json file or using the command line argument --target es6. This way, the output will match the input exactly.

It's important to note that while your code may still run without issues, the compiler throws an error during compilation to alert you of any mistakes:

for(let i = 0; i < 1; i++) {
    alert(i);
}
alert(i); // compile error: cannot find name 'i'

Answer 2

However, it's not that simple. The compiler essentially translates the "let" command to a "var" command. Why does this mechanism exist? Is Typescript optimized for use with classes only?

The compiler defaults to transpiling to ES3, where the let keyword is not supported. Therefore, the emitter has to generate code using ES3-compatible syntax, hence replacing let with var.

If you prefer the output to use the let keyword, you need to switch to targeting ES6—by setting "target": "es6" in your tsconfig.json file or using the command line argument --target es6. This way, the output will match the input exactly.

It's important to note that while your code may still run without issues, the compiler throws an error during compilation to alert you of any mistakes:

for(let i = 0; i < 1; i++) {
    alert(i);
}
alert(i); // compile error: cannot find name 'i'

Answer 3

Answer №2

When comparing the effects of var and let in this scenario, it is noted that they produce similar outcomes. However, var tends to be slightly faster on most JS engines. Therefore, TypeScript automatically optimizes performance by converting instances of let to var.

In a different example, the transformation from let to var results in more than just a simple switch:

for (let i = 0; i < 3; i++) {
  setTimeout(function() { alert(i); });
}

Interestingly, in this case, using let versus var produces contrasting outputs. While let would display 1 2 3, utilizing var would show 3 3 3. To delve deeper into the ES6 introduction of the let keyword, further exploration can be done here:

https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Instructions/let

Answer 4

When comparing the effects of var and let in this scenario, it is noted that they produce similar outcomes. However, var tends to be slightly faster on most JS engines. Therefore, TypeScript automatically optimizes performance by converting instances of let to var.

In a different example, the transformation from let to var results in more than just a simple switch:

for (let i = 0; i < 3; i++) {
  setTimeout(function() { alert(i); });
}

Interestingly, in this case, using let versus var produces contrasting outputs. While let would display 1 2 3, utilizing var would show 3 3 3. To delve deeper into the ES6 introduction of the let keyword, further exploration can be done here:

https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Instructions/let

Answer 5

Answer №3

They may appear the same at first glance, but there is a subtle distinction in how they function within a code block.

LET

function theDifference(){
    for(let emre = 0; emre < 10; emre++){
    // The variable emre is only accessible within this for loop
    }

// The variable emre is NOT accessible outside of the for loop
}

VAR

function theDifference(){
    for(var emre = 0; emre < 10; emre++){
    // The variable emre can be accessed within this for loop
    }

// The variable emre can also be accessed outside of the for loop
}

Answer 6

They may appear the same at first glance, but there is a subtle distinction in how they function within a code block.

LET

function theDifference(){
    for(let emre = 0; emre < 10; emre++){
    // The variable emre is only accessible within this for loop
    }

// The variable emre is NOT accessible outside of the for loop
}

VAR

function theDifference(){
    for(var emre = 0; emre < 10; emre++){
    // The variable emre can be accessed within this for loop
    }

// The variable emre can also be accessed outside of the for loop
}

Are 'const' and 'let' interchangeable in Typescript?

Answer №1

Answer №2

Answer №3

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