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There's one method in JavaScript that is really powerful and ideal for building a calculation, and

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that's the evil function which evaluates JavaScript script code represented as a string.

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So we see that we've got if we do eval two plus two, we're going to have to put a four.

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We get eval, we can see that a string, we can evaluate it and so on and so on.

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So we get this output one and this is the one that we're after.

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We want to get that first part where we're actually evaluating the content.

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And as we saw before, we essentially do have a value that we're evaluating.

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So if we go into here and if we do eval and don't forget to wrap it in the brackets and we have that

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value, we can see that JavaScript can calculate that out.

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So even if these are strings and string values, using eval gives us a lot of power and gives us the

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ability to calculate that out.

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So what we want to do is we want to launch the evaluation, but only on certain keys.

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We don't want every Keat that gets pressed to evaluate out on because that's not going to work properly.

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So we need to make sure that we're checking to see if the correct keys and that would be any one of

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the keys where we've got the equals sign or any one of these calculation keys operators that we're using

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within our keyboard.

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And that's where we want to apply the evaluation.

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So that means that we need to search Ahrari to check to see if it's included within our array.

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And there's a great example of the includes.

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So array includes function in JavaScript and returns a bullying value.

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So this one is ideal for what we need, where we can check to see if an array and we know we've got

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that operator array that we set up earlier where we're going to check to see if our array to do a condition,

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if operator includes and we're looking for the value that's contained within that text.

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So the might value value.

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And if it does, then that's when we can invoke our operator and we got to move this one out and then

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do our evaluation of the content.

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So take my calculator and use the eval and evaluate the contents that are contained within that output

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stream.

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So now one, two, three.

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And we do plus three and we do.

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Plus we see that the calculation is done.

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And every time we click one of these operators, we see that we're outputting that value.

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The problem comes in when we try to add two operators together and then we throw an error.

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And that's because we're evaluating we've got a plus and then we're doing a minus.

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So it's expecting a no numeric value there and it's not expecting to have a plus and a minus.

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So that's not going to work.

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And we need to apply some conditions in order to check for that to see if the last value is one of the

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operators.

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So we'll take care of that in the upcoming lesson.

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So for now, add in the ability to check through the array using that includes and also use evaluate

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in order to evaluate that string content mathematically.