Closure CavesISSUE #3 of 61

parameters · default · rest

LambdaVSThe Mutator

Lambda saysParameters can have defaults and a rest parameter that gathers extra arguments.

Give defaults inline: function greet(name = "guest"). A rest parameter (...args, always last) collects any extra arguments into an array. Destructure parameters for "named arguments": function f({x, y}). These make function signatures self-documenting and flexible.

Power-ups you unlock

Default params: name = "guest"
Rest param ...args (must be last)
Destructure params for named arguments
Defaults trigger only when the arg is undefined

The Mutator attacks — common mistakes

Rest parameter not in last position
Defaults referencing later parameters
Passing null and expecting the default (only undefined triggers it)

Boss battleWrite a function with a default parameter and a rest parameter.

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>function tag(label='item', ...vals){ return label+': '+vals.join(','); }
document.getElementById('o').textContent=tag('nums',1,2,3)+'\n'+tag();</script></body></html>

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