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Javascript understanding errors part 2 common error examples

scdan0624 profile image SCDan0624 ・2 min read

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In part 1 we went over some common syntax and reference errors that you may come across as a beginning programmer. In part 2 we will go over some more common errors along with some less common but equally important ones.

Forgetting to Close Parenthesis on a Function Call

When you create a function that does not take any arguments, a common error is to forget the parenthesis when you call that function:

function printNumberFive(){
  let x = 2
  let y = 3
  return x + y;
}

let notFive = printNumberFive; // will set notFive equal to a function but not 5
let five = printNumberFive(); // will set five equal to return x + y of function
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Passing Arguments in the Wrong Order

Another common error when dealing with functions is passing/calling the arguments in the incorrect order:

function groceryList (array,name){
  return `Hi my name is ${name} and these are my groceries I need to buy ${array}`
}

groceryList("Dan",["Bacon","Eggs","Milk"])
//'Hi my name is Bacon,Eggs,Milk and these are my groceries I need to buy Dan'
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Just remember the arguments must be called in the same order that they are passed in:

function groceryList (array,name){
  return `Hi my name is ${name} and these are my groceries I need to buy ${array}`
}

groceryList(["Bacon","Eggs","Milk"],"Dan")

//'Hi my name is Dan and these are my groceries I need to buy Bacon,Eggs,Milk'
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Off By One Error When Indexing

Off by one errors typically occur when looping over indices or targeting a specific index in a string or array. Javascript indices start at zero and a common mistake is to assume they start at 1, leading to the index you are targeting to be off by one:

let myArray = [1,2,5,10,20,40]

for(var i = 0; i <= myArray.length; i++){
  console.log(myArray[i])
}

// Loops one to many times at the end because there are only 5 indices but i <= myArray.length will attempt to print out 6 indices.

for(var i = 1; i < myArray.length; i++){
  console.log(myArray[i])
}

// Does not print the first number in the array

for(var i = 0; i < myArray.length; i++){
  console.log(myArray[i])
}

// Correctly prints all the numbers in the array

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Conclusion

Knowing these common errors can be a huge time saver. If there are other common errors I missed feel free to comment below, I would love to hear them.

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