This leaves it as undefined, and you can use it for comparisons later.

Declare a variable, but never assign a value to it. Declare a new variableĭeclare a variable, but do not initialize it: let realUndefined Ĭonsole.log(variable = realUndefined) // true As a consequence, if you mistype either of them it just silently misbehaves, which is the worst kind of error. There is a magic string, and also there is no error if the variable is not declared. It works in every situation, and can not be tampered with. This is a surprisingly lax yet correct way to check for undefined: let variable Ĭonsole.log(typeof variable = 'undefined') // true let undefined = 'a' Ĭonsole.log(variable = undefined) // false While you can not overwrite it in modern browsers, you can shadow it. The undefined is defined not as a keyword, but as a property of the global object. It correctly filters out nulls but is still prone to another problem. let variable = null Ī stricter way is to use the strict equality check: let variable It filters out 0, '', and NaN, but still returns true for null. Variable = undefined let variable Ĭonsole.log(variable = undefined) // true Falsy values also include 0, '', and NaN, so it gives false positives for them. But for checking only for undefined, it falls short. In everyday programming, it is usually a superior way, as it catches nulls and other problematic values. The other ways Check for falsy valuesĬheck if the value is falsy: if (variable) It is not immediately obvious what this code does for someone who is not familiar with this approach. The downside? It does not contain the word undefined. It is even three characters shorter than undefined. It can not be overridden, can not be shadowed, and does not produce any false positives. If you look into some libraries, they use this approach. To check for undefined that works every time, compare with void 0: let variable Ĭonsole.log(variable = void 0) // true So don't rush and refactor everything just yet, but let it be a cautionary tale how messed up a seemingly simple everyday problem can be. I didn't think much about it, but when I started researching this topic, I was surprised by the number of approaches to achieve this simple goal, and that most of them are broken in subtle ways.įortunately, most ways are broken only if you deliberately try to break them.

To check if a variable is undefined is so commonplace I hardly write any programs without it.

Javascript is undefined