# JS Safe Get

实现一个安全的读取 JS 属性

相关用例

```javascript
  const obj = {a: {aa: {aaa: 2}}, b: 4};

  get(obj, 'a.aa.aaa'); // 2
  get(obj, ['a', 'aa', 'aaa']); // 2

  get(obj, 'b.bb.bbb'); // undefined
  get(obj, ['b', 'bb', 'bbb']); // undefined

  get(obj.a, 'aa.aaa'); // 2
  get(obj.a, ['aa', 'aaa']); // 2

  get(obj.b, 'bb.bbb'); // undefined
  get(obj.b, ['bb', 'bbb']); // undefined

  get(obj.b, 'bb.bbb', 42); // 42
  get(obj.b, ['bb', 'bbb'], 42); // 42

  get(null, 'a'); // undefined
  get(undefined, ['a']); // undefined

  get(null, 'a', 42); // 42
  get(undefined, ['a'], 42); // 42

  const obj = {a: {}};
  const sym = Symbol();
  obj.a[sym] = 4;
  get(obj.a, sym); // 4
```

思路

读取参数，将属性获取形成一个数组，逐一读取，若中途无法访问则返回 default, 无异常会返回最后的值

```javascript
function get(obj, propsArg, defaultValue) {
  if (!obj) {
    return defaultValue;
  }
  var props, prop;
  if (Array.isArray(propsArg)) {
    props = propsArg.slice(0);
  }
  if (typeof propsArg == 'string') {
    props = propsArg.split('.');
  }
  if (typeof propsArg == 'symbol') {
    props = [propsArg];
  }
  if (!Array.isArray(props)) {
    throw new Error('props arg must be an array, a string or a symbol');
  }
  while (props.length) {
    prop = props.shift();
    if (!obj) {
      return defaultValue;
    }
    obj = obj[prop];
    if (obj === undefined) {
      return defaultValue;
    }
  }
  return obj;
}
```


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