I'm reading through the ES5 spec and got a bit confused about the following piece:
8.6 The Object Type
... There are two kinds of access for named (non-internal) properties: get and put, corresponding to retrieval and assignment, respectively.
Can someone clarify what this phrase refers to exactly (what is its meaning?)?
I have only seen the named accessor properties get and set.
The spec is saying that putting and getting are two actions that can happen to properties. Objects have the internal methods [[Put]] and [[Get]] that correspond to those actions.
[[Put]] is an internal method, present within every object, that stores values in an object's properties. Any time you perform an assignment operation on a property, you cause the environment to perform a [[Put]].
[[Set]] is the internal property of an accessor property descriptor. This is what you define when you define a set setter function of an accessor property. (Note: [[Set]] exists on property descriptors, not on objects. Not all properties have a [[Set]], but all objects do have a [[Put]].)
If you attempt to perform a [[Put]] on an object property which is an accessor property, that operation will invoke the property's [[Set]] function, per step 5 of the [[Put]] algorithm:
If IsAccessorDescriptor(desc) is true, then
- a. Let setter be desc.[[Set]] which cannot be undefined.
- b. Call the [[Call]] internal method of setter providing O as the this value and providing V as the sole argument.
In short, the function [[Set]] only applies to accessor properties, while [[Put] can apply to either accessor or data properties.