日本語のみで絞り込む
2010/5/6 -The best way is to check the type, because undefined / null / false are a tricky thing in JS. So: if(typeof obj !== "undefined") { // obj is ...
2025/4/17 -Ben Nadel discovers that undefined values in JavaScript do not soft equal false or 0. Which makes expressions involving the safe navigation ...
Comparing with Double Equals. If we use the double equal sign operator ( == ), null is equal to null , and undefined is equal to undefined .
2026/1/10 -You can use undefined and the strict equality and inequality operators to determine whether a variable has a value. In the following code, the ...
2025/2/7 -In JavaScript, undefined coerces into NaN when coerced into a number, like when used in a comparison (contrast that with null , which ...
2019/10/4 -One of the most common errors we encounter in JavaScript is the undefined error when trying to compare two values. Let me give you an example so ...
2025/11/12 -Treat any comparison with undefined/null except the strict equality === with exceptional care. Don't use comparisons >= > < <= with a variable ...
2016/10/20 -There are some style guides that basically say you always should use === , but you can use == in order to check for null or undefined at the ...
2024/3/31 -In the strictest sense, null represents a value intentionally defined as "blank," and undefined represents a lack of any assigned value. null ...
2022/7/11 -How to Check if a Variable is Undefined in JavaScript with Direct Comparison. One of the first methods that comes to mind is direct comparison.