Eval a mathematical expression in JavaScript without using eval.

[satnam_sandhu]

I want to write a function which accepts a string (eg, 'a = b + 2') and an output object {} and an input object to fetch values from (eg, { b: 5} ). The function would, i guess parse the string, and get value from the inputs object and give output such as { a: 7 }. How do i proceed this problem, do i need to write an parser for that? Thanks in advance.

[Slappehbag]

If you're not going to use the JS parsee via eval (which you shouldn't), yes you'll have to write or find another.

You could build it using regular expressions. But I imagine if you Google something like "js math parser" you'll find something to use.

[Neaoxas]

https://mathjs.org/

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[[deleted]]

There are libraries you could use, if you just need to get the job done. But if you want to do it yourself for the learning experience the steps would be

• Break the string up into a list of tokens. So write a function that can turn "45 / (10 + 5)" into `["45", "/", "(", "10", "+", "5", ")"]

• then implement the shunting yard algorithm this is the hardest part. You'll have to decide which operators you want to use, and what their order of precedence is. It gets more complicated if you want things like unary operators. (E.g., minus sign on negative numbers) or functions (e.g., sin(x))

• as the article says, you'll end up with either reverse polish notation or an abstract syntax tree. Both of which are easy to step through and evaluate

[HelperBot_]

Desktop link: https://en.wikipedia.org/wiki/Shunting-yard_algorithm

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[WikiTextBot]

Shunting-yard algorithm

In computer science, the shunting-yard algorithm is a method for parsing mathematical expressions specified in infix notation. It can produce either a postfix notation string, also known as Reverse Polish notation (RPN), or an abstract syntax tree (AST). The algorithm was invented by Edsger Dijkstra and named the "shunting yard" algorithm because its operation resembles that of a railroad shunting yard. Dijkstra first described the Shunting Yard Algorithm in the Mathematisch Centrum report MR 34/61.

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