TypeError: can't access property "x" of "y"

The JavaScript exception "can't access property" occurs when property access was operated on undefined or null values.

Message

TypeError: Unable to get property {x} of undefined or null reference (Edge)
TypeError: can't access property {x} of {y} (Firefox)
TypeError: {y} is undefined, can't access property {x} of it (Firefox)
TypeError: {y} is null, can't access property {x} of it (Firefox)

Examples:
TypeError: x is undefined, can't access property "prop" of it
TypeError: x is null, can't access property "prop" of it
TypeError: can't access property "prop" of undefined
TypeError: can't access property "prop" of null

Error type

TypeError.

What went wrong?

The property access was operated on undefined or null value.

Examples

Invalid cases

// undefined and null cases on which the substring method won't work
var foo = undefined;
foo.substring(1); // TypeError: x is undefined, can't access property "substring" of it

var foo = null;
foo.substring(1); // TypeError: x is null, can't access property "substring" of it

Fixing the issue

To fix null pointer to undefined or null values, you can use the typeof operator, for example.

if (typeof foo !== 'undefined') {
  // Now we know that foo is defined, we are good to go.
}