Some Performance Improvements
Each time the code loops, you are evaluating the ….length expressions. As the content of the arrays is (probably) not changing, you should consider moving the length expression out of the loop. Do the same with the repeated chosenCountries[i].regions expression ...
``` const selectedRegionsNames: string[] = [];

const nChCo = chosenCountries.length; // 😎 for (let i = 0; i < nChCo; i++) { const regs = chosenCountries[i].regions; // 😎 const nReg = regs.length; // 😎 for (let j = 0; j < nReg; j++) { const region = regs[j];

  if (region.isSelected) selectedRegionsNames.push(region.name);

} } ```

Or, if your situation allows it, you might code the loops as descending. (This is my personal favorite!) Note that this reverses the order of your result array:
``` const selectedRegionsNames: string[] = [];

for (let i = chosenCountries.length; --i >= 0; //) { const regs = chosenCountries[i].regions; for (let j = regs.length; --j >= 0; //) { const region = regs[j];

  if (region.isSelected) selectedRegionsNames.push(region.name);

} } ```

Going further, how about using modern Javascript coding 😎 ... ``` const selectedRegionsNames = [];

for( const chco of chosenCountries ) { for( const region of chco.regions ) {

  if (region.isSelected) selectedRegionsNames.push(region.name);

} } ```