Feature request: extend built-in functions

[vasile-baluta]

Hello.
We need to add some more built-in functions.

One would be the diff between two numbers. Another function would make a number negative. We can contribute with the code ourselves if it is ok.

[kesor]

I am missing a function that will parse inline JSON, like the from_string function in here - https://github.com/Kintyre/jmespath/blob/7e185c/bin/jpath.py#L15

[glenveegee]

We've implemented this in a new typescript version if you're interested to check it out. The implementation maintains validation and interpreter scoping within the function. It also helps to add any number of custom functions as a series of plugins to jmespath. See https://github.com/nanoporetech/jmespath-ts

Here's a snippet from the unit test:

  it('register a customFunction', () =>  {
      expect(() => search({
        foo: 60,
        bar: 10
      }, 'divide(foo, bar)')
      ).toThrow('Unknown function: divide()');


      jmespath.registerFunction('divide', function(resolvedArgs) {
          const [dividend, divisor] = resolvedArgs;
          return dividend / divisor;
        },
        [{ types: [jmespath.TYPE_NUMBER] }, { types: [jmespath.TYPE_NUMBER] }]
      )

      expect(search({
        foo: 60,
        bar: 10
      }, 'divide(foo, bar)')
      ).toEqual(6)
  });