feat: resolve get array element count

[jacksjm]

Closes #768

• [x] Running yarn lint does not trigger any linter errors
• [x] The test of the method you have fixed is passing
• [x] You have written a new skeleton method for someone else to work on!
• [x] You have written tests to accompany your skeleton method

[jacksjm]

@ktilcu sorry i'm resolve another issue because my others PR's not be approved in repositories :disappointed:

[jacksjm]

generate issue #811