(1*0.4)(1*2)(1*0.2) is valid or invalid?

[inapeace0]

I doubt that this expression is valid with math.js module.

(1*0.4)(1*2)(1*0.2)

Actually, the original expression is (1*0.4)+(1*2)+(1*0.2), and of course, it's valid with this module.

[gwhitney]

Well, if you type that exact expression into the "demo" at mathjs.org, you will see that it responds with a value of 0.16 and no indications of any problem or error. So indeed, it is a valid expression.

[inapeace0]

Yes, I see. With math.js, it's detected it's valid.
(1*0.4)(1*2)(1*0.2) = (1*0.4)*(1*2)*(1*0.2) = 0.16
But why I asked it is that it must be detected as invalid one in my project. In other words, there must be operators(+, -, *, /) between two operators ')' and '('.

[gwhitney]

As far as I know, there is not currently any mechanism in mathjs to disallow implicit multiplication, i.e. require an explicit * symbol to multiply juxtaposed subexpressions. The parse() function does have an "options" argument, so if you need it, you could submit a PR implementing a "multiplication" option with values 'allowImplicit' or 'onlyExplicit' (say) to control this. But yes, as far as I know it would currently require a code change to disallow implicit multiplication in expressions.

[josdejong]

If you want to disable implicit multiplication right now, you can do the following: parse the expression with math.parse, and then traverse the created expression tree to see if there is an occurrence of an OperatorNode which has a property implicit: true, and if so, throw an error. If all is ok you can evaluate the parsed expression.

Docs: https://mathjs.org/docs/expressions/expression_trees.html