Aha-moments-in-JS

Just some Aha! moments in learning Javascript.

• Object is a function, not an object.

  Similarly, Array is function, too.

• When used with one argument, Object.create() behaves the same as:

  function object(o) {
    function F() {}
    F.prototype = o
    return new F()
  }

• There are two types of properties in javascript objects, one is data property, and the other is accessor property.

  when defined through Object.defineProperty, both data property and accessor property have these keys as descriptor:
  configurable, enumerable, value, writable,
  but an accessor property has get and set

• ES6 minor changes:

1, formalized the __proto__ property.

2, Safe integers

Number.MAX_SAFE_INTEGER = 2^53
Number.MIN_SAFE_INTEGER = -2^53
Number.isSafeInteger(num)

3, New Math methods:

Math.acosh(x), Math.asinh(x), Math.atanh(x)

Math.cbrt(x) The cubed root of x

Math.log1p(x): The natural logarithm of 1 + x, Math.log2(x), Math.log10(x)

Math.trunc(x): Remove the fration from a float.

4, UTF-16 support:
codePointAt() method.

• The executor in new Promise() is executed immediately.

  Say,

  var p = new Promise(function(resolve, reject) {
    console.log('Yeah');
    resolve(42);
  })

  will log the Yeah right after this declaration.

• About Object.freeze() and Object.seal()

  freeze: prevents new properties from being added to it; prevents existing properties from being removed; and prevents existing properties, or their enumerability, configurability, or writability, from being changed. The method returns the object being frozen.

seal: preventing new properties from being added to it and marking all existing properties as non-configurable. Values of present properties can still be changed as long as they are writable.

Notice that freeze is shallow, you have to do a deepFreeze to make it a real constant. And there is no error throwing out when you trying to change the freezed object’s property.

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