(program):42 Syntax error @ 'Thing Checker'!
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JSHINT output:
script: (W104) 'let' is available in ES6 (use esnext option) or Mozilla JS extensions (use moz) on line: 18 at character: 5
script: (W104) 'let' is available in ES6 (use esnext option) or Mozilla JS extensions (use moz) on line: 19 at character: 5
script: (W104) 'let' is available in ES6 (use esnext option) or Mozilla JS extensions (use moz) on line: 20 at character: 5
script: (W104) 'let' is available in ES6 (use esnext option) or Mozilla JS extensions (use moz) on line: 21 at character: 5
script: (W104) 'let' is available in ES6 (use esnext option) or Mozilla JS extensions (use moz) on line: 23 at character: 5
script: (W104) 'let' is available in ES6 (use esnext option) or Mozilla JS extensions (use moz) on line: 28 at character: 13
script: (W104) 'let' is available in ES6 (use esnext option) or Mozilla JS extensions (use moz) on line: 32 at character: 13
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SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    at Function (native)
    at c (<anonymous>:41:115)
    at eval (eval at <anonymous> (unknown source), <anonymous>:1:25)
    at eval (native)
    at n (<anonymous>:72:175)
    at ja (<anonymous>:41:158)
    at ma (<anonymous>:83:93)
    at b (<anonymous>:83:318)(anonymous function) @ (program):42
(program):42 Uncaught SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode