JavaScript is a powerful and flexible language, but it also has some quirks that can surprise even experienced developers. Understanding these strange behaviors can help you write more robust and bug-free code. In this article, we'll explore some of the most notable oddities in JavaScript.
JavaScript automatically converts types in certain situations, which can lead to unexpected results.
console.log(1 + '1'); // '11' - Number 1 is coerced to a string console.log(1 - '1'); // 0 - String '1' is coerced to a number console.log(true + true); // 2 - true is coerced to 1 console.log('' == 0); // true - Empty string is coerced to 0 console.log([] == 0); // true - Empty array is coerced to 0
NaN stands for "Not-a-Number" and is used to represent a value that is not a valid number. Interestingly, NaN is not equal to itself.
console.log(NaN === NaN); // false console.log(Number.isNaN(NaN)); // true - Correct way to check for NaN
The typeof operator can return some unexpected results.
console.log(typeof null); // 'object' - This is a long-standing bug in JavaScript console.log(typeof []); // 'object' - Arrays are technically objects in JavaScript console.log(typeof function(){}); // 'function' - Functions have their own type
Adding two arrays together can produce surprising results due to type coercion.
console.log([] + []); // '' - Both arrays are coerced to empty strings console.log([] + {}); // '[object Object]' - Empty array is coerced to empty string, empty object is coerced to string '[object Object]' console.log({} + []); // 0 - Here, {} is interpreted as an empty block
JavaScript uses floating-point arithmetic, which can lead to precision issues.
console.log(0.1 + 0.2); // 0.30000000000000004 console.log(0.1 + 0.2 === 0.3); // false
The == operator performs type coercion before comparison, which can lead to unexpected results. It’s generally better to use the strict equality operator (===).
console.log('' == false); // true console.log(0 == false); // true console.log('' == 0); // true console.log(null == undefined); // true
Variables declared with var are function-scoped, not block-scoped, which can lead to unexpected behavior.
if (true) { var x = 5; } console.log(x); // 5 - x is available outside the block for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1000); } // Output: 3 3 3 - Because var is function-scoped, the same i is referenced
In the browser, this refers to the global window object in non-strict mode. This can cause some surprising behaviors.
function foo() { console.log(this); // window (in browser) } foo(); const bar = { method: function() { console.log(this); // bar object } }; bar.method(); const baz = bar.method; baz(); // window (in browser)
JavaScript is a versatile and powerful language, but it's important to be aware of its quirks and idiosyncrasies. By understanding these strange behaviors, you can avoid common pitfalls and write more reliable code. Keep exploring and experimenting with JavaScript to deepen your understanding of these and other interesting aspects of the language.
Do you know other quirks in Javascript? Write them in the comments.
Do you know the basic methods in Javascript? Check out my article here to learn more!
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