Today #Day132 of #365DaysOfCode, Learning Essential JavaScript Interview Questions.
31. What would the following code return?
console.log(typeof typeof 1);
string
typeof 1 will return "number" and typeof "number" will return string.
console.log(typeof 1); // number
console.log(typeof '1'); // string
console.log(typeof number); // undefined
console.log(typeof 'number'); // string
console.log(typeof typeof 1); // string
32. What will be the output of the following code: & Explain your answer. How could the use of closures help here?
for (var i = 0; i < 5; i++) {
setTimeout(function () {
console.log(i);
}, i * 1000);
}
The code sample shown will not display the values 0, 1, 2, 3, and 4 as might be expected; rather, it will display 5, 5, 5, 5, and 5.
The reason for this is that each function executed within the loop will be executed after the entire loop has completed and all will therefore reference the last value stored in i, which was 5.
Closures can be used to prevent this problem by creating a unique scope for each iteration, storing each unique value of the variable within its scope, as follows:
for (let i = 0; i < 5; i++) {
(function (x) {
setTimeout(function () {
console.log(x);
}, x * 1000);
})(i);
}
This will produce the presumably desired result of logging 0, 1, 2, 3, and 4 to the console.
In an ES2015 context, you can simply use let instead of var in the original code:
for (let i = 0; i < 5; i++) {
setTimeout(function() { console.log(i); }, i * 1000 );
}
Conclusion
Learned and practiced Essential JavaScript Interview Questions involving topics like typeof and set timeout.
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