Node.js
Mark-sweep
Learn mark-sweep and mark-compact algorithms.
Mark-sweep
Mark-sweep is the garbage collection algorithm used for the old generation. It has two steps: mark — starting from roots, it marks all reachable objects; sweep — it deletes unmarked objects. This handles circular references correctly.
A markSweep(roots, allObjects) function models this: it finds objects reachable from roots and discards the rest. new Set(roots) builds the reachable set; allObjects.filter(o => reachable.has(o)) keeps only reachable ones. Example: markSweep(['a', 'b'], ['a', 'b', 'c', 'd']) returns 'a,b'. c and d are unreachable; they are swept. This model shows mark-sweep with a deterministic filter.
The reachable set
The essence of mark-sweep is checking whether an object is reachable from the roots. new Set(roots) builds the reachable set. reachable.has(o) tells whether an object is in the set. This is a simple model of the mark step.
Example: const reachable = new Set(['a', 'b']). reachable.has('a') is true, reachable.has('c') is false. allObjects.filter(o => reachable.has(o)) keeps only ['a', 'b']. .join(',') yields the string 'a,b'. This shows how mark-sweep decides which objects live: those reachable from roots live, the rest are swept. The output is a,b.
Exercise Write the mark-sweep function
Write function markSweep(roots, allObjects). Define const reachable = new Set(roots). Return allObjects.filter(o => reachable.has(o)).join(','). Print console.log(markSweep(['a', 'b'], ['a', 'b', 'c', 'd'])).
Show example solution
function markSweep(roots, allObjects) {
const reachable = new Set(roots);
return allObjects.filter(o => reachable.has(o)).join(',');
}
console.log(markSweep(['a', 'b'], ['a', 'b', 'c', 'd']));
Test yourself
-
Match each concept to its description
- Mark
- Marks reachable objects
- Sweep
- Deletes unmarked objects
- Roots
- Starting points
- Set
- Set data structure
-
What does markSweep(['a'], ['a','b','c']) return?
markSweep(['a'], ['a','b','c'])
Show answer
'a' — Only 'a' is reachable from the root; 'b' and 'c' are swept.
-
Complete the filter call
return allObjects.____(o => reachable.has(o)).join(',');
Show answer
return allObjects.filter(o => reachable.has(o)).join(',');
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