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1 2 3 4 5 6 7 |
for (var i = 0; i < 4; i++) { // global scope setTimeout(() => console.log(i)); } for (let i = 0; i < 4; i++) { // block scope setTimeout(() => console.log(i)); } |
The output of the above for loops is 4 4 4 4 and 0 1 2 3 Explanation: Due to the event queue/loop of javascript, the setTimeout callback function is called after the loop has been executed. Since the variable i is declared with the var keyword it became a global variable and the value was equal to 4 using iteration when the time setTimeout function is invoked. Hence, the output of the first loop is 4 4 4 4.
Whereas in the second loop, the variable i is declared as the let keyword it becomes a block scoped variable and it holds a new value(0, 1 ,2 3) for each iteration. Hence, the output of the first loop is 0 1 2 3.
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