JS Common Mistakes
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Assignment vs Comparison
= assigns a value while == or === compares one; accidentally writing if (x = 5) inside a condition silently assigns 5 to x and always evaluates truthy, a classic hard-to-spot bug.
Example: Assignment vs Comparison
let x = 0;
if (x = 5) { // mistake: assignment instead of comparison
console.log("Always runs, x is now", x);
}Variable Scope Mistakes
Declaring a variable with var inside a loop or block doesn't scope it the way let does, which can lead to every closure in a loop capturing the same final value instead of its own.
Example: Variable Scope Mistakes
var funcs = [];
for (var i = 0; i < 3; i++) {
funcs.push(() => console.log(i));
}
funcs[0](); // 3, not 0 - var is not block-scopedArray and Object Mistakes
Mutating an array or object you didn't mean to change — for example, calling .sort() (which mutates in place) when you intended to keep the original order — is a frequent source of subtle bugs.
Example: Array and Object Mistakes
const original = [3, 1, 2];
const sorted = original.sort(); // mutates in place!
console.log(original); // [1, 2, 3] - original changed tooAsync and Function Mistakes
Forgetting to await a Promise, or mixing .then() chains with async/await inconsistently, can cause code to run out of order or silently swallow an error the catch block never sees.
Example: Async and Function Mistakes
async function getData() {
const result = fetchData(); // forgot await
console.log(result); // logs a Promise, not the resolved value
}
function fetchData() { return Promise.resolve(42); }
getData();Type and Conversion Mistakes
Comparing values with == instead of === can trigger unexpected type coercion (like '' == 0 being true), while assuming typeof null is object misdirects some type-checking logic.
Example: Type and Conversion Mistakes
console.log('' == 0); // true - unexpected coercion with ==
console.log(typeof null); // "object" - a long-standing quirkChapter Quiz — Complete all 9 topics to unlock
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