How do I fix 'undefined is not a function' and similar JavaScript errors?

Read the error properly first — that alone solves most of them. A JavaScript error tells you the type of problem, the thing that caused it, and the exact file and line. Most people skim it, panic at the red, and start changing things randomly, which turns a two-minute fix into an hour. What the common ones actually mean: - `undefined is not a function` / `x is not a function` — you called something that isn't a function. Usually a typo in the method name (`.lenght()`), calling a property that isn't a method, or the object being a different type than you assumed (calling an array method on something that isn't an array). - `Cannot read property 'x' of undefined` (or `of null`) — the single most common JavaScript error. You tried to access `something.x` but `something` doesn't exist yet. Almost always: data hasn't loaded from an API yet, an object key is spelled differently than you think, or a function returned nothing. - `x is not defined` — the variable doesn't exist in this scope. Typo, missing import, wrong scope, or used before declaration. - `Unexpected token` — a syntax error, usually a missing bracket, brace or comma near the reported line (check the line *above* too). The systematic method that replaces random editing: 1. Read the error type and the message. Say out loud what it claims. 'It says `user` is undefined at line 42.' 2. Go to that exact line and file from the stack trace. The top line of the trace is your code most of the time. 3. Print the thing it complained about, just before the failing line: `console.log(user)`. Now you know whether it's undefined, an empty object, or something unexpected. This one step resolves the majority of these errors. 4. Work backwards. If it's undefined, where was it supposed to be set? Trace to the source — an API response, a function return, a parameter that wasn't passed. 5. Fix the cause, not the symptom. Adding `?.` or a null check silences the crash, but if the data was supposed to be there, you've hidden a real bug rather than fixed it. Use optional chaining when a value is *legitimately* sometimes absent (still loading, optional field), not to mute a mystery. The mindset shift that matters: errors are the language telling you precisely what's wrong, in a specific location. They feel hostile at first and become genuinely useful once you slow down and read them. A developer who reads stack traces carefully debugs several times faster than one who doesn't — and it's a skill you can acquire in a week of deliberately reading every error before touching anything.