I will try to explain this by taking an example.
create a file main.ts
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function printData(){
for(var i=0; i<5; i++){
console.log(i);
}
console.log('last value:' + i);
}
printData();
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Now compile and run the compiled code like:
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tsc main.ts | node main.js
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Output will be:
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last value: 5
here value of i at the end is 5.
This is the issue of declaring a variable with var keyword. we have declared value of i inside the for block but it is also available outside the for block or it is available inside the whole function body.
if you declare the variable with let keyword.like:
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function printData(){
for(let i=0; i<5; i++){
console.log(i);
}
console.log('last value:' + i);
}
printData();
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It will immediately give a compilation error say 'can not find name i' and you will catch this error at compile time. This is the beauty of Typescript.
Now if you still compile this file:
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tsc main.ts
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It will give the error : 'can not find name i'.
However it will still generate the file main.js with the JavaScript code.
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function printData(){
for(var i=0; i<5; i++){
console.log(i);
}
console.log('last value:' + i);
}
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Because this JavaScript code is perfectly valid JavaScript code so now if you run this like:
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node main.js
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it will give the same output:
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last value: 5
so using let doesn't stop the compilation but we can catch the issue at compile time.
