chrome · JavaScript
The message
TypeError: items.map is not a function
What it means
The name exists but it is not a function — if it did not exist at all, the message would talk about undefined instead. Typical causes are something you took for an array that is really an object or a NodeList, an API that returns {data: [...]} rather than the array itself, or the wrong shape of default export from a module. Print it before you theorise: one console.log(typeof items, items) settles half of these on the spot. A NodeList or Set only needs Array.from(items); but if it was {data: [...]}, wrapping it is wrong and items = res.data is the fix.
The fix
There is no one-line command for this. The explanation says what to look at instead.
- Printed by
- chrome
- JavaScript
- 15
Browser and Node errors usually tell you what broke and not why the value became what it was — you read undefined, it was not a function, it was not JSON — so the place to fix is upstream of the line that threw, and silencing just that line with ?. and default values makes the same problem reappear further away in a form that is harder to recognise.
Reading an error message
- Read from the first line down. The lower you go the more it is about the tool’s internals; the cause is usually at the top.
- If there is a file and a line number, start there — not the top stack frame, but the topmost line that names a file you wrote.
- Search the message verbatim, but strip your own paths and variable names first; those are what stop the search from matching.
- The same condition is worded differently across tool versions. If results look wrong, add the version number to the query.
- Before pasting a fix, check what it throws away. Some of these cannot be undone.
Common questions
Q. What does “TypeError: items.map is not a function” mean?
The name exists but it is not a function — if it did not exist at all, the message would talk about undefined instead. Typical causes are something you took for an array that is really an object or a NodeList, an API that returns {data: [...]} rather than the array itself, or the wrong shape of default export from a module. Print it before you theorise: one console.log(typeof items, items) settles half of these on the spot. A NodeList or Set only needs Array.from(items); but if it was {data: [...]}, wrapping it is wrong and items = res.data is the fix.
Q. How do I fix it?
There is no one-line command. The explanation above says what to look at instead.
Q. Which tool prints this?
chrome. It sits under JavaScript, and the message runs to 6 words.