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# expr

Tiny property path utilities, including path parsing and metadata and deep property setters and getters

    npm install property-expr

## Use

Setters and getters:

```js
let expr = require('property-expr')
let obj = {
  foo: {
    bar: ['hi', { buz: { baz: 'found me!' } }]
  }
}

let getBaz = expr.getter('foo.bar[1]["buz"].baz')
let setBaz = expr.setter('foo.bar[1]["buz"].baz')

console.log(getBaz(obj)) // => 'found me!'
setBaz(obj, 'set me!')
console.log(obj.foo.bar[1].buz.baz) // => 'set me!'
```

### `getter(expression, [ safeAccess ])`

Returns a function that accepts an obj and returns the value at the supplied expression. You can create a "safe" getter, which won't error out when accessing properties that don't exist, reducing existance checks befroe property access:

```js
expr.getter('foo.bar.baz', true)({ foo: {} }) // => undefined
//instead of val = foo.bar && foo.bar.baz
```

### `setter(expression)`

Returns a function that accepts an obj and a value and sets the property pointed to by the expression to the supplied value.

### `expr(expression, [ safeAccess], [ paramName = 'data'])`

Returns a normalized expression string pointing to a property on root object
`paramName`.

    expr.expr("foo['bar'][0].baz", true, 'obj') // => "(((obj.foo || {})['bar'] || {})[0])"

### `split(path) -> Array`

Returns an array of each path segment.

```js
expr.split("foo['bar'][0].baz") // [ "foo", "'bar'", "0", "baz"]
```

### `forEach(path, iterator[, thisArg])`

Iterate through a path but segment, with some additional helpful metadata about the segment. The iterator function is called with: `pathSegment`, `isBracket`, `isArray`, `idx`, `segments`

```js
expr.forEach('foo["bar"][1]', function(
  pathSegment,
  isBracket,
  isArray,
  idx,
  segments
) {
  // 'foo'   -> isBracket = false, isArray = false, idx = 0
  // '"bar"' -> isBracket = true,  isArray = false, idx = 1
  // '0'     -> isBracket = false, isArray = true,  idx = 2
})
```

### `normalizePath(path)`

Returns an array of path segments without quotes and spaces.

```js
expr.normalizePath('foo["bar"][ "1" ][2][ " sss " ]')
// ['foo', 'bar', '1', '2', ' sss ']
```

### `new Cache(maxSize)`

Just an utility class, returns an instance of cache. When the max size is exceeded, cache clears its storage.

```js
var cache = new Cache(2)
cache.set('a', 123) // returns 123
cache.get('a') // returns 123
cache.clear()

cache.set('a', 1)
cache.set('b', 2) // cache contains 2 values
cache.set('c', 3) // cache was cleaned automatically and contains 1 value
```

### CSP

This pacakge used to rely on `new Function` to compile setters and getters into fast
reusable functions. Since `new Function` is forbidden by folks using Content Security Policy `unsafe-eval`
we've moved away from that approach. I believe that for most cases the perf hit is not noticable
but if it is in your case please reach out.

If you really want to use the old version require `property-expr/compiler` instead