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The other form (called for in) enumerates the property names (or keys) of an object. On each iteration, another property name string from the object is assigned to the variable.
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Douglas Crockford (JavaScript: The Good Parts: The Good Parts)
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The converted string is assigned to a variable. In this case, it's the same variable whose string is being converted,
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Mark Myers (A Smarter Way to Learn JavaScript: The new approach that uses technology to cut your effort in half)