6.5. Debugging Logic Errors

We can debug runtime and syntax errors using the error messages produced. Logic errors, however, do not generally produce error messages. This sometimes makes them tougher to debug.

While we can't provide a step-by-step approach that applies to every possible logic error, we can give you some solid strategies. Two such strategies---using debugger tools and writing tests---will be covered in future lessons. In this section, we start with a basic and effective way to debug logic errors.

6.5.1. Printing Values

When your code runs but doesn't produce the expected results, it is important to check the values of the variables being used.

Let's look at a program that has a logical bug.

const input = require('readline-sync');

let degreesC = input.question('Temp in degrees C:');
let degreesK = degreesC + 273.15;

console.log('Degrees K:', degreesK);

repl.it

This program asks the user for a temperature in degrees celsius and attempts to convert it to degrees Kelvin. Degrees Kelvin differs from degrees celsius by 273.15. So if we enter 100 (in celsius) we should see a converted value of 373.15 (in Kelvin). However, running the program as-is and entering 100 gives the message:

Temp in degrees C:  100
Degrees K: 100273.15

This is clearly incorrect. But the program does not generate an error, so it is not immediately clear what the issue is. To figure it out, we'll use console.log to see what the values of key variables are when the program runs.

Let's first make sure that the degreesC variable looks like it should by adding a console.log statement just after we create this variable.

const input = require('readline-sync');

let degreesC = input.question('Temp in degrees C: ');
console.log(degreesC);
let degreesK = degreesC + 273.15;

console.log('Degrees K:', degreesK);

Running this with an input of 100 gives the output:

Temp in degrees C:  100
100
Degrees K: 100273.15

The second line is the value of degreesC, which appears to be correct. But the final answer is still incorrect, so we need to keep digging for more information.

Looking at the line in which we set degreesK, we see that we use degreesC as a numeric value in our calculation. Let's see what the data type of degreesC is. In the end, we want it to be a number.

const input = require('readline-sync');

let degreesC = input.question('Temp in degrees C: ');
console.log(typeof degreesC);
let degreesK = degreesC + 273.15;

console.log('Degrees K:', degreesK);

Running this with an input of 100 gives the output:

Temp in degrees C:  100
string
Degrees K: 100273.15

That's it! The variable degreesC has the value 100, but it is a string rather than a number. So when we set degreesK with the formula degreesC + 273.15, we are actually performing string concatenation instead of addition: "100" + 273.15 is "100273.15".

We can fix our program by converting the user's input to the number data type.

const input = require('readline-sync');

let degreesC = input.question('Temp in degrees C: ');
degreesC = Number(degreesC);
let degreesK = degreesC + 273.15;

console.log('Degrees K:', degreesK);

Running this with an input of 100 gives the output:

Temp in degrees C:  100
Degrees K: 373.15

Note that after debugging we removed all of our console.log statements. Be sure to do the same when using this debugging technique.