lisp.js | A lisp interpretter in javascript

This is because there is a tiny bug in your code. When you call the evaluate(lambdaBody, lambdaEnvironment) at line:134, you provide lambdaBody as the first argument, which is the expr argument in the 'function evaluate(expr, env)' function (line:138). During your evaluation you consume it with tempArg = expr.shift(); function and get out of the loop when its empty. For the second time, your lambdaBody is unfortunately an empty array.

To fix it, you should change the evaluate function not to modify its arguments (e.g. create a copy and consume the copy)

I suggest you fix the bug in the evaluate function, but a sample that really works is to change line 134 to deep copy lambdaBody as below: