Closure CavesISSUE #37 of 61

errors · throwing · custom error classes

LambdaVSThe Mutator

Lambda saysThrow errors with throw; create meaningful ones by extending the Error class.

throw new Error("message") signals a problem and unwinds to the nearest catch. Throw Error objects (not strings) so you get a stack trace. Define custom types by extending Error (class ValidationError extends Error) so callers can catch and branch on the kind of failure.

Power-ups you unlock

throw new Error("msg") signals failure
Throw Error objects, not strings
Extend Error for custom, catchable types
Set err.name for clearer logs

The Mutator attacks — common mistakes

Throwing strings (no stack trace)
Catching a generic Error when a specific type would help
Throwing for normal control flow

Boss battleDefine a custom error class and throw it, then catch and read its name.

Example code

<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre id="o"></pre>
<script>class ValidationError extends Error{constructor(m){super(m);this.name='ValidationError';}}
try{ throw new ValidationError('bad input'); }catch(e){ document.getElementById('o').textContent=e.name+': '+e.message; }</script></body></html>

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