What this snippet does
If an interface has an index signature, all explicitly defined properties must match the index signature's type.
Expected output
Compile Error: Property 'length' of type 'number' is not assignable to 'string' index type 'string'.
Why practise typing this
Typing objects and interfaces code builds muscle memory for the symbols and indentation TypeScript uses most, which prose-based typing tests never cover. DevType measures your words per minute and accuracy on this snippet and tracks the individual characters you mistype, so later lessons can target them.
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