JavaScript Patterns 3.7 Primitive Wrappers
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时间：2015年06月11日
Primitive value types: number, string, boolean, null, and undefined.
// a primitive number var n = 100; console.log(typeof n); // "number" // a Number object var nobj = new Number(100); console.log(typeof nobj); // "object"
One reason to use the wrapper objects is when you want to augment the value and persist state. Because primitives are not objects, they cannot be augmented with properties.
// primitive string var greet = "Hello there"; // primitive is converted to an object // in order to use the split() method greet.split(‘ ‘)[0]; // "Hello" // attemting to augment a primitive is not an error greet.smile = true; // but it doesn‘t actually work typeof greet.smile; // "undefined"
When used without new, wrapper constructors convert the argument passed to them to a primitive value:
typeof Number(1); // "number" typeof Number("1"); // "number" typeof Number(new Number()); // "number" typeof String(1); // "string" typeof Boolean(1); // "boolean"
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