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JavaScript: Predicates

Remember the isInfant() function from the last lesson:

const isInfant = (age) => age < 1;
console.log(isInfant(3));
false

This kind of function is called a predicate. Predicate functions answer a question and always (without exception!) return either true or false.

Predicates usually have handy names in every language for simplicity of analysis. In JavaScript, predicates generally begin with is, has, or can, though it's not limited to just those words. Examples:

  • isInfant() - "is he an infant?"
  • hasChildren() - "does he have children?"
  • isEmpty() - "is it empty?"
  • hasErrors() - "are there any errors?"

Only a function returning a boolean can be considered a predicate.


Let's write one more predicate function. It takes a string and checks if it's the word 'Castle':

const isCastle = (type) => type === 'Castle';

console.log(isCastle('Sea'));
false

Instructions

Write the function isMister() that takes a string and checks if it's the word 'Mister'.

Examples:

isMister('Mister'); // true
isMister('Miss');   // false
The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

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In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

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Tips

Definitions

  • Predicate - is an expression that answers a yes or no question with a boolean.


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