An if statement in Node.js has the following structure.

if (condition) {
  <do something>
} else if (condition) {
  <do something>
} else {
  <do something>
}

For example.

const foo = "Hello";

if (foo == "Hello") {
  console.log("foo equals Hello");
} else if (foo == "World") {
  console.log("foo equals World");
} else {
  console.log("foo does not equal Hello or World");
}

If the "foo" var (variable) or const (constant) is defined

if (typeof foo === 'undefined') {
  console.log("foo is undefined")
} else {
  console.log(`foo is defined and is a ${typeof foo}`)
}

If the foo var (variable) or const (constant) contains a value

if (foo.length === 0) {
  console.log("foo is empty (contains no value)")
} else {
  console.log(`foo = ${foo}`)
}

If the foo var (variable) or const (constant) contains "hello"

const foo = "Hello World"

if (foo.toLowerCase().includes('hello')) {
  console.log(`${foo} includes hello`)
} else {
  console.log(`${foo} does NOT include hello`)
}

If the foo var (variable) or const (constant) contains "hello" or "world"

const foo = "Hello World"

if (/hello|world/.test(foo.toLowerCase())) {
  console.log(`${foo} includes hello or world`)
} else {
  console.log(`${foo} does NOT include hello or world`)
}

If the foo var (variable) or const (constant) is greater than or equal to or less than or equal to

if (foo > 100) {
  console.log(`${foo} is greater than 100`)
} else {
  console.log(`${foo} is less than 100`)
}

if (foo >= 100) {
  console.log(`${foo} is greater than or equal to 100`)
} else {
  console.log(`${foo} is less than 100`)
}

if (foo < 100) {
  console.log(`${foo} is less than 100`)
} else {
  console.log(`${foo} is greater than 100`)
}

if (foo <= 100) {
  console.log(`${foo} is less than or equal to 100`)
} else {
  console.log(`${foo} is greater than 100`)
}

Here is how you can string together two (or more) if statements.

if ((foo > 100) && (foo < 100)) {
  console.log(`${foo} is greater than 100 and less than 200`)
} else {
  console.log(`${foo} is less than 100 or greater than 200`)
}

If the /tmp/foo.txt file exists

import fs from "fs"

fs.readFile('/tmp/foo.txt', (err, data) => {
  if (!err && data) {
    console.log(data)
  } else {
    console.error(err)
  }
})